This invention relates to pumps which can supply a charge of compressed gas on demand, such as for guns which use a charge of compressed air to fire a pellet.
Air guns have a wide following because laws limiting their use are not as restrictive as for powder guns, and air guns are relatively inexpensive to shoot. Air gun shooting is an Olympic sport, and hunting with an air gun removes much of the danger inherent with powder guns while retaining and enhancing the challenge.
Air guns fall into three major groups:
1. Pump guns: These guns use one or more strokes from a pumping device to store a charge of compressed air in a firing chamber. The required effort to charge the gun increases with each pump as the stored pressure builds. The power of the gun depends on the strength of the shooter because the relatively low mechanical advantage of the pumping mechanism. Most of these guns completely expel the air charge when fired. On firing, the pellet is initially exposed to the full pressure of the compressed air, but the available pressure falls rapidly as the pellet accelerates down the gun barrel. These guns usually have moderate power, driving a pellet at about 500 feet per second. U.S. Pat. No. 4,572,152 to Olofsson, et al., discloses an air gun which uses a floating piston to store compressed air in an auxiliary chamber. The purpose of the floating piston is to augment firing pressure by moving to displace air in the firing chamber when the gun is fired. However, with the gun disclosed in the Olofsson, et al. patent, the compressed air stored in the auxiliary chamber is limited to that provided by one stroke of the pump, and the pressure in the auxiliary chamber can never be greater than the pressure in the firing chamber.
2. Spring guns: These guns use a single stroke of a lever to compress a steel spring. On firing, the spring drives a relatively heavy piston that causes a rapid increase in air pressure within a firing chamber. The firing chamber is directly connected to the gun barrel. The pellet is held in the gun barrel by a seal until the air pressure in the chamber reaches an optimum point. When this happens, the air pressure overcomes the holding ability of the seal and drives the pellet down the barrel. The piston also continues to displace air in the firing chamber, thereby helping to maintain pressure on the pellet. This method has replaced multi-stroke pumping as the most common air gun mechanism. Only one stroke of the lever accomplishes the entire cocking procedure. Thus, a spring gun usually takes less time to place into action than a multi-stroke gun. By maintaining a more constant force on the pellet as it travels down the barrel, the imparted energy may be twice that available with a conventional pneumatic multi-pump gun. However, the drawback of a spring gun is that only one stroke of the lever is available to compress the spring. The most powerful spring guns require strength beyond the limit of many people. Moreover, the spring imposes a practical limit on the amount of energy that can be stored. At least one model has replaced the steel spring with a compressed air “spring.” The compressed air in the “spring” is not expended but is re-compressed with the gun's lever. The air spring can store more energy in a smaller space, but considerable work must be expended by the shooter.
3. Pre-charged guns: These guns use a gas charge that is pre-packaged and inserted into the gun with little expenditure of energy by the user. The most common guns of this type use a small container of liquid carbon dioxide to power the gun. Each firing of the gun uses a portion of the stored liquid, which rapidly vaporizes on firing. A method gaining popularity transfers compressed air from a storage bottle into a relatively large storage vessel attached to the gun. For example, air from a diver's scuba tank or similar storage vessel is transferred into the storage vessel on the gun through a high-pressure hose and clamp assembly. The gun gets multiple shots from charges provided by the air in the storage vessel, but the accuracy of the gun diminishes with the loss of available pressure until the storage vessel is refilled. Some carbon dioxide (CO2) guns use small canisters available at hardware stores. These guns are moderately powerful, but also suffer from accuracy problems with the loss of pressure in the canister. Guns which use compressed air from large detached tanks can store more energy and suffer less in accuracy lost between shots. However, the detached tank (such as a scuba tank) is heavy and cumbersome.
In summary, multiple-pump air guns are limited by the strength of the user, and the initial strokes are time consuming for the amount of useful energy transferred to the storage chamber. Spring guns use one quick pull of a lever and achieve efficiency with the available energy, but are limited by the strength of the individual loading the gun. Guns which use a pre-charged vessel of compressed gas must have the vessel in close proximity to the gun, and cannot rely on precision repeat performance with each shot.
Maximum muzzle energy for the three types of guns is about 11.5 foot-pounds for the best multi-pump guns, about 25 foot-pounds for the best spring guns, and about 30 foot-pounds for the best pre-charged gun using air from a scuba tank.
Convenient power is the goal of air guns. With more power the pellet trajectory is flatter, accuracy is enhanced, and more energy is delivered at the point of impact.